1. Field of the Invention
The invention is concerned with the field of power electronics. It is based on a method for operating a power electronic circuit arrangement according to the preamble of claim 1.
2. Discussion of Background
Such a method is disclosed in German Patent Application 196 15 855.9, which is not a prior publication. What is involved in that case is a power electronic circuit arrangement in which an improved approximation of the sinusoidal output voltage is intended to be achieved by providing at least two series-connected power converters which are driven in such a way that the intermediate circuit voltages of the power converters can be combined as desired.
If the intermediate circuit voltage of the first power converter is chosen to be greater than that of the second power converter, then the second power converters constitute so-called fine stages whose output voltage can be added to or subtracted from that of the first power converter. The switches of the power converters are driven by a pulse duration-modulated pulse train, which is generated for example by comparison of a reference oscillation with a carrier oscillation. In the event of a maximum drive level, that is to say maximum output voltage and maximum frequency, the majority of the output voltage is intended to be formed by the slowly pulsed first power converter. The latter is preferably also furnished with powerful GTOs, while the second power converter may be equipped e.g. with IGETs. The switching frequency of the first power converter should be as low as possible in order that the switching losses do not become too large. In the event of a low drive level, that is to say low frequency and low output voltage, it may thereby happen, however, that the power of the second power converter becomes negative, that is to say that said converter outputs power. This corresponds to energy feedback from the second power converter. However, this is undesirable; the circuit arrangement becomes less expensive when it does not have to be capable of taking up energy that has been fed back.